Construction of the Kaifeng Outer City Wall and the Form of the City Gate of Northern Song Dynasty

On the base of the city wall of the Wudai period, the outer city wall of Kaifeng of Northern Song Dynasty formed internal and external structure with maintenance of many times. On the wall are some defending facilities such as Mamian, barbican, dilou,zhanpeng,etc. The city gate consists of double ones, treble barbicans and water gates. The outer city make Kaifeng city of Northern Song Dynasty looks like a military castle.